1. Field of the Invention
The present invention is directed to a measuring arrangement for acquiring a signal corresponding to the respiratory motion of a patient, and in particular to such an arrangement wherein a pneumatic pressure signal is obtained corresponding to the respiratory motion which is converted into an intensity-modulated optical signal.
2. Description of the Prior Art
It is known when obtaining a tomogram using magnetic resonance imaging (MRI or NMR) techniques to monitor the respiratory motion of the examination subject during certain types of exposures, so that the generation of the image can be gated, synchronized with the respiratory motion, to avoid motion artifacts in the image. For this purpose, it is known to obtain a pressure signal corresponding to the motion caused by respiration using a pneumatic respiratory belt. The pneumatic pressure signal is then conducted via a pressure conduit to a location remote from the belt, wherein the signal is converted into an electrical signal by a capacitive transducer or by a piezoelectric pressure sensor.
A measurement apparatus of this type is described in U.S. Pat. No. 4,324,259, wherein the pressure signal is supplied to a capacitive transducer wherein one of the capacitor plates is mounted on a flexible membrane which is deformed by the pressure signal. This causes the spacing between the plates to vary, thereby resulting in a capacitance signal which is modulated by the pressure signal. If this type of device were to be used in the environment of a magnetic resonance imaging tomography apparatus, the transducer portion of the device would necessarily have to placed at a considerable distance from the examination subject wearing the respiratory belt, the subject being disposed in the magnetic resonance imaging apparatus. This is because, due to the influences of the strong high frequency fields on electronic components, it is necessary to place all electrically conductive materials at a location sufficiently remote from the fields so that the components are not affected by the fields. The necessity of using such a long pressure conduit between the belt and the transducer unavoidably results in deterioration of the pressure signal due to the inactive volume of the conduit. This decreases the sensitivity of the measuring arrangement, and may result in a faulty correlation between the derived electrical signal and the actual respiratory motion.
A respiratory motion sensor specifically designed for use in a magnetic resonance imaging environment is disclosed in U.S. Pat. No. 4,664,129. This known arrangement includes a belt having a buckle consisting of two mating parts which are mechanically connected so as to be relatively movable. In one embodiment, a light transmitter is disposed in one of the parts, and a light receiver is disposed in the other part. A light polarizer is disposed in front of the light receiver, so that movement due to respiration will cause the light from the transmitter to be polarized by different amounts, since the light will arrive at different directions due to the motion. By analyzing the degree of polarization, a signal corresponding to movement of the examination subject is obtained. In another embodiment of the invention, the transmitter and receiver are disposed in the same belt part, and the other part contains a mirror which reflects the light from the transmitter back to the receiver. A mechanical, pneumatic signal is thus not used at all in this arrangement.